1. Field of the Invention
The present invention relates to the field of radio communications. More particularly, the present invention relates to a transmission apparatus and a receiving apparatus that adopt the OFDM (Orthogonal Frequency Division Multiplexing) scheme.
2. Description of the Related Art
The OFDM scheme, which is getting attention in the field of radio communications, is a technology for realizing good signal transmission even under a multipath transmission environment by arranging plural carriers (sub-carriers) at intervals of a frequency such that the plural carriers are orthogonal to each other. In a transmission apparatus in this scheme, as shown in FIG. 1 briefly, a source signal is generated in a signal generation part based on transmission information. The signal is converted to a parallel signal series by a serial-parallel conversion part (S/P), processed by an inverse fast Fourier transform part (IFFT part), and converted to a serial signal series by a parallel-serial conversion part (P/S). Then, a guard interval is added in a guard interval adding part (GI), and the signal is amplified by a power amplifying part (PA) so as to be transmitted as an OFDM signal by radio. As is generally known, the guard interval is a copy of an end part of the transmission symbol. On the other hand, in the receiving apparatus, as shown in FIG. 2, the guard interval is removed from a received signal by a guard interval removing part, the received signal is converted to parallel signals by a serial-parallel conversion part (S/P), the parallel signals are transformed by a fast Fourier transform part (FFT part), and are converted to a serial signal by a parallel-serial conversion part (P/S), and further demodulated in a signal detection part so that the transmission information is obtained.
In the OFDM scheme, since various sub-carriers are used, compared to the average amplitude, a signal with very large peak amplitude may be generated after the inverse fast Fourier transform part in some cases as shown in FIG. 3. The ratio of a possible maximum peak power to an average power is referred to as PAPR (Peak to Average Power Ratio). Generally, the maximum peak power may become the number of all sub-carriers times greater than average power.
On the other hand, as shown in FIG. 4, the power amplifier (PA) has a linear region that provides linear input/output characteristics and a non-linear region that provides non-linear input/output characteristics. For outputting a transmission signal having small distortion, it is desirable that the power amplifier (PA) operate in the linear region. If the power amplifier (PA) operates in the non-linear region, there may arise problems such as degraded transmission quality, and unwanted emission to outside of the band. When the PAPR is large, the power amplifier is being used not only in the linear region but also in the non-linear region. A power amplifier having a wide liner region may be used, but it sacrifices power efficiency. Therefore, it is desirable that the PAPR of the transmission signal be small.
In a first document (M. Friese, “On the degradation of OFDM-signals due to peak-clipping in optimally predistorted power amplifiers, Proc. of GLOBCOM '98, pp. 939-944, November 1998), a so-called pre-distortion scheme is adopted, in which inverse characteristics of the distortion are weighted on a signal input to an amplifier, which enables amplifying the signal linearly if it is below the saturation level.
In a second document (X. Li and L. J. Cimini, “Effects of clipping and filtering on the performance of OFDM”, IEEE Commun. Lett., vol. 2, no. 5, pp. 131-133, May 1998), a large peak value is clipped for reducing PAPR.
In addition, a technology called PTS (Partial Transmit Sequence) is known, in which a combination of phase rotation amounts is selected for a transmission signal from among plural combinations of phase rotation amounts that are set for each sub-carrier, so that the phase is rotated for each sub-carrier to reduce PAPR (refer to the following documents:                L. J. and N. R. Sollenberger, “Peak-to-Average power ratio reduction of an OFDM signal using partial transmit sequences”, IEEE Commun. Lett., vol. 4, no. 3, pp. 86-88, March 2000 (third document);        S. H. Muller and J. B. Huber, “A Novel Peak Power Reduction Scheme for OFDM”, Proc. of PIMRC '97, pp. 1090-1094, 1997 (fourth document);        G. R. Hill, Faulkner and J. Singh, “Reducing the peak-to-average power ratio in OFDM by cyclically shifting partial transmit sequences” Electronics Letters, vol. 36, No. 6, 16 Mar. 2000 (fifth document)).        
FIG. 5 shows an example of a transmission apparatus adopting the PTS scheme. FIG. 6 shows an example of a receiving apparatus adopting the PTS scheme. In the examples shown in the figures, a signal series generated in the signal generation part is divided into two signal series. On each of the two signal series, serial-parallel conversion and inverse Fourier transform are performed. Each of inverse Fourier transform parts IFFT1,2, each having N input-output points, receives N/2 signals and N/2 null symbols from the serial-parallel conversion part. The phase rotation amount control part determines proper phase rotation amounts {θ1, θ2, . . . } so that one of them is commonly supplied to each multiplying part. Outputs from the inverse Fourier transform part are synthesized with proper weights in the synthesizing part. The signal group, on which inverse Fourier transform has been performed, and that has been synthesized is converted to a serial signal in a parallel-serial conversion part (P/S), the guard interval is added to the serial signal in the guard interval part (GI), so that the signal is transmitted from the antenna. In the receiving side, as shown in FIG. 6, the phase rotation amount is compensated for when the received signal is demodulated.
However, the pre-distortion scheme clips a part of a signal having a peak power level exceeding a saturation region of the amplifier.
In the method in which an unwanted peak power value is clipped, orthogonality among sub-carriers collapses, so that interference between symbols increases and receiving quality may be degraded.
With regard to the PTS method, it is necessary to perform complex multiplication for each signal series when supplying a weight to each signal so that complexity increases. This problem is disadvantageous especially for consumed power, circuit size and the like. It can be considered to decrease kinds of the weights to be supplied to the signal series so as to decrease calculation work load for generating the transmission signal. However, by reducing the kinds of the weights, effectiveness of weight adjustment is greatly limited.
Further, the weight to be supplied is used merely for adjusting the phase. Assuming that a signal having a peak power before being synthesized in the synthesizing part is Sp, and that a part of the OFDM signal by which a weight (exp(jθ)) is multiplied is S, an output from the synthesizing part is Sp+exp(jθ)S, wherein the phase rotation amount θ is determined such that the peak amplitude of the synthesized signal becomes small. However, when the amplitude of the signal S is very small, it is difficult to reduce the amplitude of the synthesized signal by adjusting the phase rotation amount θ.
In addition, it is difficult to easily obtain the shift amount in the conventional cyclic shift method and it is difficult to easily obtain the phase rotation amount in the conventional PTS method, so that control for reducing the peak is difficult in the conventional technology. Further, calculation work load related to control increases as the number of signal series obtained by the dividing inverse Fourier transform part increases, which is disadvantageous especially for a small mobile terminal.